The circuit protective conductor (increasingly
called the 'c.p.c.') is a system of conductors joining together
all exposed conductive parts and connecting them to the
main earthing terminal. Strictly speaking, the term includes
the earthing conductor as well as the equipotential bonding
conductors.

The circuit protective conductor can take
many forms, such as:

1. - a separate conductor which
must be green/yellow insulated if equal to or less than
10 mm2 cross-sectional area.

2. - a conductor included in a sheathed
cable with other conductors

3. - the metal sheath and/or armouring
of a cable

4. - conducting cable enclosures
such as conduit or trunking

5. - exposed conductive parts, such
as the conducting cases of equipment

This list is by no means exhaustive and
there may be many other items forming parts of the circuit
protective conductor as indicated in {Fig 5.10}. Note that
gas or oil pipes must not be used for the purpose, because
of the possible future change to plastic (non-conducting)
pipes.

Fig 5.10 Some types of
circuit protective conductor

Fig 5.11 Protective connection
for socket outlet in conduit system

It is, of course, very important that the
protective conductor remains effective throughout the life
of the installation. Thus, great care is needed to ensure
that steel conduit used for the purpose is tightly jointed
and unlikely to corrode. The difficulty of ensuring this
point is leading to the increasing use of a c.p.c. run inside
the conduit with the phase conductors. Such a c.p.c. will,
of course, always be necessary where plastic conduits are
used. Where an accessory is connected to a system (for example,
by means of a socket outlet) which uses conduit as its c.p.c.,
the appliance (or socket outlet) earthing terminal must
be connected by a separate conductor to the earth terminal
of the conduit box (see {Fig 5.11}). This connection will
ensure that the accessory remains properly earthed even
if the screws holding it into the box become loose, damaged
or corroded.